In general, the construction of motor-driven injection molding machine broadly comprises a mold clamping mechanism A and an injection mechanism B, as shown in FIG. 6.
The mold clamping mechanism A is composed of a movable platen 95 having a movable mold half MM fixed thereto, a stationary platen 96 having a stationary mold half SM fixed thereto, a rear platen 97, a toggle mechanism 98 for moving the movable platen 95 toward or away from the stationary platen 96 to effect mold clamping or opening, a mold clamping motor 99 for actuating the toggle mechanism 98, etc.
The injection mechanism B is composed of an injection screw 5 and an injection cylinder 2 for injecting a molten resin into a cavity of a mold, which is formed of the movable mold half MM and the stationary mold half SM clamped together, and metering it, a motor 36 for injection as a drive source for advancing the injection screw 5 with respect to the injection cylinder 2 to effect injection, a motor 35 for screw rotation as a drive source for rotating the injection screw 5 in the injection cylinder 2 to effect metering and kneading, etc. The rotation of the motor 36 for injection is transmitted to a ball screw 10 through a pulley 13, and the rotation of this ball screw 10 is converted into a force to press a pusher plate 6 by means of a ball nut 9. Thereupon, the pusher plate 6, guided by guide rods 4, moves straight, so that the screw 5 is actuated for translation. On the other hand, the rotation of the motor 35 for screw rotation is transmitted to the screw 5 through a pulley 20, to drive the screw.
In FIG. 6, symbol BS designates a base. While the mold clamping mechanism A is fixed to the base BS, the whole injection mechanism B is allowed to move toward or away from the mold clamping mechanism A so that the distal end (nozzle) of the injection cylinder 2 can be made either to come into contact or move away from the mold.
Referring now to FIG. 5, the injection mechanism B will be described in detail. A front plate 1 and a rear plate 3 are fixed integrally to each other by means of the guide rods 4, which serve also as tie rods. The injection cylinder 2 is fixed to the front plate 1. The injection screw 5 is attached to the pusher plate 6 so as to be rotatable and immovable in the axial direction. This pusher plate 6 is slidably mounted on the guide rods 4 through bushings 7.
The ball nut 9 is fixed to the side face of the pusher plate 6 which faces the rear plate 3 through a load cell 8, which detects detecting a resin reaction force, so as to be rotatable with respect to the pusher plate 6 but immovable in the injection-axis direction (i.e., axial direction of the injection screw). The ball nut 9 mates with the ball screw 10 that is attached to the side of the rear plate 3 so as to be rotatable and immovable in the axial direction. The ball screw 10 is supported by means of three angular bearings 11, 11 and 12 so as to be rotatable with respect to the rear plate 3 and immovable in the axial direction. Inner and outer rings of each of the bearings 11, 11 and 12 are fixed, respectively, to the outer periphery of the ball screw 10 and the inner periphery of a through hole that is formed in the center of the rear plate 3. Among these bearings, each of the two angular bearings 11 includes an inner ring and an outer ring for supporting a resin reaction force that acts on the inner ring from forward. Also, the angular bearing 12 includes an inner ring and an outer ring for supporting a force that acts on the inner ring from behind.
The axis of the ball screw 10 is in line with the axis of the injection screw 5. In the description to follow, therefore, this common axis will be referred to as an "injection axis."
The pulley 13 for injection is fixed to the end portion of the ball screw 10 which projects rearward from the rear plate 3. When the rotatory force of the motor for injection (not shown), which is fixed in the vicinity of the rear plate 3, is transmitted to the pulley 13, the ball screw 10 rotates to drive the ball nut 9 for feeding. The driven ball nut 9 presses the pusher plate 6 through the load cell 9, thereby moving the injection screw 5 in the axial direction.
As this is done, a frictional force between the ball screw 10 and the ball nut 9 is transmitted to the pusher plate 6 through the ball nut 9 and the load cell 8, causing a force to act so as to rotate the pusher plate 6 around the injection axis. As a result, the bushings 7 in the pusher plate 6 engage only one side of the guide rods 4 so that sliding resistances are generated between the pusher plate 6 and the guide rods 4.
Since the load cell 8 detects the force of pressure of the pusher plate 6, which slides on the guide rods 4, on the ball screw 10, it detects the sum of the substantial resin reaction force that acts on the tip of the injection screw 5 and the sliding resistances between the pusher plate 6 and the guide rods 4. The sliding resistances between the pusher plate 6 and the guide rods 4 cannot take a constant value, as it varies depending on the magnitude of the rotatory force of the ball screw 10. It is difficult, therefore, to accurately extract only the resin reaction force that acts on the tip of the injection screw 5 by simply subtracting a sliding resistance component from the detection output of the load cell 8.